1. Field of the Invention
The present invention relates to an optical amplifier and an optical communications system, and more particularly, to an optical amplifier amplifying wavelength multiplexed light, and a wavelength multiplexed light communications system.
2. Description of the Related Art
With the popularization of the Internet, an image data transmission service, etc., the amount of information transmitted via a network has been rapidly increasing, and a WDM (Wavelength Division Multiplex) optical communications system has been being introduced to cope with this phenomenon. The WDM optical communications system has been being introduced not only into a long-haul trunk system but also a metro-ring network.
In the long-haul trunk system, optical amplifiers are normally arranged at predetermined intervals, and each of the optical amplifiers amplifies wavelength multiplexed light in ALC (Automatic Level Control) mode or AGC (Automatic Gain Control) mode according to circumstances. Here, the ALC mode is an operation mode in which the output of an optical amplifier is held to be a predetermined level, whereas the AGC mode is an operation mode in which the gain of an optical amplifier is held to be a predetermined value.
Each of the optical amplifiers normally operates in the ALC mode in order to stabilize its optical level diagram. When the number of wavelengths of wave multiplexed light increases or decreases, the operation mode of each of the optical amplifiers is switched from the ALC mode to the AGC mode according to a control signal from a terminal station. Here, the time constant (response time) of the ALC is set to several tens to several hundreds of milliseconds in order to facilitate an implementation of an ALC circuit, and to suppress the influence of PDL (Polarization Dependent Loss). In the meantime, the time constant (response time) of the AGC is set to several tens of milliseconds. “The time constant (response time) of the ALC” means, for example, a time period from when the input/output level of an optical amplifier changes till when the power of pump light or a loss in a variable attenuator is suitably adjusted to make the output level revert to a predetermined level to be held, although this meaning is not uniquely defined. In the meantime, “the time constant (response time) of the AGC” means, for example, a time period from when the input/output level of an optical amplifier changes till when the power of pump light is suitably adjusted to make the gain of the optical amplifier revert to a predetermined value to be held, although this meaning is not uniquely defined. An optical amplifier having the ALC mode and the AGC mode is recited in detail, for example, by Japanese Patent Publication No. 2000-151515.
In a metro-ring network, a plurality of optical nodes are connected in the form of a ring, and the above described optical amplifier is arranged within each of the plurality of optical nodes. Here, in the metro-ring network, the frequency of a change in the number of wavelengths of wavelength multiplexed light is high in comparison with a long-haul trunk system due to the following reasons.    (1) A path is established/released between arbitrary optical nodes depending on need.    (2) For IP over WDM suitable for an IP network, it is desirable that a protection function is provided by a WDM system. Here, considering the demand for independency from a bit rate, a method switching a wavelength in an optical layer is promising as the protection function. With this method, however, the number of wavelengths increases/decreases in a transient manner.    (3) In the future, a time-based wavelength leasing service is expected to be rendered. In this case, a path is frequently established/released between arbitrary optical nodes.
As described above, the number of wavelengths of wavelength multiplexed light frequently changes in a metro-ring network. An optical amplifier within each optical node amplifies wavelength multiplexed light while suitably switching its operation mode each time the number of wavelengths changes.
To reduce the cost of an optical communications system, the cost of components configuring an optical node or an optical amplifier must be cut down. As one embodiment, configuration using an avalanche photodiode (APD) that is relatively cheap instead of using the combination of optical pre-amplifier and PIN photodiode is known up to now.
However, at a 10-Gbps transmission speed, the dynamic range of optical receiver using APD is normally narrow, and the output level of an optical amplifier must control output power level of each channel within the narrowest possible range considering the tilt of wavelength multiplexed light (wavelength dependency of an optical level), a variation in the loss characteristic of an optical component, the impact due to a change in an external environment, etc. For example, at the 10-Gbps transmission speed, the dynamic range of a optical receiver using APD is about ±10 dB in the system using optical amplifier. Here, a variation in a loss in a demultiplexer arranged in an optical node is about ±2 dB, and also the tilt of wavelength multiplexed light is on the order of ±2 dB. Assuming that the range where an ambient temperature changes is 60 degrees, and the length of a transmission line between two nodes is 100 km, a variation of up to 1.8 dB or so occurs in an optical level. Accordingly, when the variation of the output level of an optical amplifier is large, the input power level of optical receiver goes out of its dynamic range. In result, it leads to an occurrence of a reception error.
However, with an existing optical amplifier, a variation in its output level is difficult to be suppressed due to the following reasons.    (1) An optical amplifier normally operates in the ALC mode, and in the AGC mode when the number of wavelengths of wavelength multiplexed light changes as described above. At this time, wavelength number information is notified, for example, by a control signal transmitted via each optical node. However, this control signal is normally interpreted after being converted into an electric signal at each node, reconverted into an optical signal, and transferred to the next optical node while 3R (regenerating, reshaping, and retiming) operations are performed. Accordingly, it sometimes takes several hundreds of milliseconds to several seconds from when the number of wavelengths of wavelength multiplexed light changes till when the wavelength number information reaches each optical node. In the meantime, in the ALC mode, the output level of an optical amplifier is controlled to become a predetermined level which corresponds to the number of wavelengths of wavelength multiplexed light. At this time, the number of wavelengths is notified with the above described wavelength number information. Accordingly, the optical amplifier operates to maintain the output level which corresponds to the number of wavelengths before the change for the time period from when the number of wavelengths changes till when the wavelength number information is notified (several hundreds of milliseconds to several seconds in the above provided example). As a result, the output level of each wavelength varies. For example, if the number of wavelengths of wavelength multiplexed light increases from three to five, an optical amplifier which operates in the ALC mode amplifies wavelength multiplexed light by assuming that the three wavelengths are multiplexed, for a time period until receiving the wavelength number information. Therefore, the output level of each of the wavelengths significantly drops.    (2) In the AGC mode, the power of pump light is adjusted according to a change in an input level, so that an output power changes to maintain a predetermined gain. However, as the time constant of the AGC mode, a value longer than the response time of an optical amplifier is normally used. Here, “the response time of an optical amplifier” means, for example, a time period from when the power of pump light supplied to an amplification medium of the optical amplifier changes till when an excited state corresponding to the power of the pump light is obtained in the amplification medium, although this meaning is not uniquely defined. Accordingly, if the optical amplifier operates in the AGC mode, the power of pump light cannot follow a change in the input level. As a result, a state where a suitable gain cannot be obtained occurs in a transient manner. For example, if total input power drops suddenly due to a decrease in the number of wavelengths of wavelength multiplexed light, the optical amplifier that operates in the AGC mode amplifies wavelength multiplexed light by assuming that the state before the total input power drops continues, for a time period required to suitably adjust the power of pump light. Therefore, in this case, the output level of each wavelength rises temporarily.    (3) In the AGC mode, the power of pump light is controlled to make the ratio of an input level to an output level constant. However, a signal gain deviates from a target value due to ASE (Amplified Spontaneous Emission) light generated in an optical amplification medium (such as an erbium-doped fiber).    (4) In the AGC mode, the power of pump light is controlled to make the ratio of an input level to an output level constant. Therefore, if input light is suspended in a protection operation, etc. of a communications system, etc., a gain control system becomes unstable. Accordingly, a surge (here, a phenomenon that the output level of an optical amplifier temporarily becomes much higher than regular output level) can possibly occur, when the optical amplifier makes a transition from the state where the input light is suspended to the state where the signal light is input.
As described above, the output level of an existing optical amplifier sometimes varies if its input level changes (including the case where the number of wavelengths of wavelength multiplexed light changes).